The present invention relates generally to cardiac pacers, and pacing methods and more particularly to a battery-powered implantable cardiac pacer and method in which the minimum pacing pulse energy required to reliably stimulate a patient's heart is determined and pacing pulse output level is automatically adjusted in accordance therewith to maximize battery life.
Cardiac pacers are beneficially used in the treatment of a variety of cardiac disorders. The operating requirements placed on a cardiac pacer often vary widely in accordance with both the nature of the particular disorder to be treated, and with the specific needs of an individual patient. It is therefore a common and well known practice to construct pacers such that the various pacer operating parameters can be adjusted over a considerable range. This is particularly advantageous since the economic benefits of large volume production can then be realized without restricting a physician's ability to tailor pacer operation according to a patient's individual needs.
While the ability to control various aspects of pacer operation provides many advantages, the flexibility thus afforded requires that consideration be given to a number of factors in selecting the pacer operating parameters. It is of course vital that the pacing output energy be sufficient to reliably stimulate the heart. Yet, battery current drain should be minimized in order to avoid the need for premature battery replacement surgery. It is also desirable that normally occurring changes in the patient's own physiological parameters will not necessitate physician readjustment of pacer operation.
Previously, pacer output energy level was set by a physician in accordance with a patient's "capture threshold" determined at the time of pacer implantation. This threshold, which represents the minimum pacing energy required to reliably stimulate a patient's heart, provided a useful starting point in selecting the pacer output energy level. However, since capture threshold typically increases over time as scar tissue grows at the interface between the implanted cardiac pacer leads and the myocardium, the usual present procedure is to set the pacer output level to a minimum of three or four times the initially measured capture threshold. While this procedure assures reliable cardiac stimulation, battery current drain is clearly above the theoretical optimum minimum level which would result if the pacer could at all times be operated at, or slightly above, the actual capture threshold existing at any particular moment.
Previously, however, capture threshold could only be measured indirectly through a laborious and time consuming procedure involving several manipulations of pacer level. In one previous external pacer, pacer level could be momentarily reduced by a predetermined constant factor in response to actuation of a push button control on the pacer housing. While this greatly increased the speed with which a physician could determine the post-implantation capture threshold, readjustment of the actual pacing output level nevertheless required the intervention of the physician. Safety dictated that the pacing level be set in the implanted pacer substantially greater than the measured capture threshold in order to assure reliable stimulation in the event the capture threshold increased substantially, before a physician could intervene. Thus, in the absence of a reliable, rapid and automatic capture threshold determination system, which could be directly incorporated into either an implantable or external cardiac pacer, any attempt to operate close to the actual capture threshold created a substantial risk that pacing reliability would be compromised as the lead-myocardium interface aged.
The present invention is directed to a cardiac pacer and method which automatically determines a patient's actual capture threshold and then sets the pacing energy level slightly above the threshold so determined. The pacer further includes a system for monitoring the sufficiency of the generated pacing pulses such that the capture threshold is redetermined, and the pacing level reset, any time it appears that the currently generated pacing pulses are not reliably stimulating the heart. Accordingly, the pacer can be arranged to operate slightly above the actual capture threshold without incurring a loss of pacing reliability. This significantly increases battery life while simultaneously avoiding the need for frequent manual readjustment of pacing pulse level.
In view of the foregoing, it is a general object of the present invention to provide a new and improved cardiac pacer and method of pacing.
It is a further object of the present invention to provide a cardiac pacer and method of pacing which minimizes battery current drain while retaining pacing functional reliability.
It is still a further object of the present invention to provide an improved cardiac pacer which operates slightly above the patient's actual capture threshold.